Eye surgery apparatus with vacuum surge suppressor

ABSTRACT

Collapse of the cornea into the cavity formed when material such as the lens is removed from an eye in the irrigation-aspiration method is minimized by incorporating a pressure responsive, variable flow resistance element in the aspiration line of the irrigation-aspiration apparatus. A preferred pressure responsive, variable resistance element is a length of thin walled tubing which is easily collapsed to smaller flow path area in response to increasing negative pressure in the aspiration line.

This invention relates to methods and means for accomplishing surgicalremoval of material from the eye with increased safety.

BACKGROUND

A variety of procedures are currently being employed to accomplishremoval of cataracts and other material from within the eye. One ofthose procedures is often referred to as an irrigation-aspirationmethod. It involves breaking, tearing or otherwise dividing the materialto be removed into small fragments and aspirating those fragments fromthe eye into a fluid flow line by which they are carried away. Fluidpressure levels in the apparatus by which that procedure is practicedare dictated by the need to preserve pressure balance within the eye.More particularly, in the case of cataract removal, it is important toprevent or minimize collapse of the cornea into the cavity formed byremoval of lens material. Difficulty in preventing such collapse hastended to discourage some surgeons from the use of theirrigation-aspiration method notwithstanding that it's successful usecan result in substantially less discomfort to patients in the postoperative period.

In the method, an incision is made on or about the corneal margin togain access to the lens. A tool in the form of a probe which houses theoutlet end of a fluid supply line and the inlet end of an aspirationline is inserted into the opening under the cornea. Some means isprovided for dividing the lens material into small fragments. The meansfor dividing eye material may comprise no more than a cutting edge at ornear the flow tube openings. More commonly an ultrasonically driven toolis included to aid in division of the material to be removed. The taskis to separate that material and divide it into small pieces, aspiratethat material from the eye and to replace the material so removed withwater, actually a balanced, salt in water solution. The opening in whichthe probe is inserted is small, 6 millimeters or less, and the openingtends to close so that a substantially closed cavity is formed as lensmaterial is removed. The water is disposed under the cornea and servesto prevent corneal collapse into the cavity. It is the water in thatcavity that is used to aspirate and carry away lens material.Accordingly the flow of water to the lens cavity must equal the flow ofwater being removed by aspiration augmented by enough water to accountfor increase in lens cavity size. That flow ratio must be maintainedwhile maintaining enough water in the lens cavity to prevent thecornea's collapse. The inner surface of the cornea is covered by a layerof irreplaceable endothelium cells. Collapse of the cornea would bringthose cells into destructive contact with the removal tool. In the caseof a sonically activated removal tool, collapse of the cornea intocontact with the tool could result in puncture of the cornea. Toaccomplish the pressure balance to prevent those catastrophes requiresprecise control of supply water pressure and aspiration pressure. Inpractice, positive supply pressure is achieved by elevating the supplywater container. Aspiration pressure is negative whereby the pressure atthe lens cavity is close to atmospheric pressure.

Two primary factors tend to upset the desired pressure level at the eye.Negative pressure in the aspiration circuit is developed by a pump inmost practical systems. Peristatic pumps are usually used but whateverthe pump form, small cyclic variations in negative pressure occur andtend to make the cornea oscillate over the cavity being formed by thelens. A greater and more troublesome pressure variation occurs when anocclusion or partial occlusion of the aspiration opening is overcome.Occlusion occurs when a piece of eye material too large to pass throughthe aspiration inlet is drawn to it. Pressure in the aspiration line isforced more negative by the aspiration pump until the blocking materialis divided or drawn into the inlet and the occlusion is cleared. Whenthat occurs, the negative aspiration pressure, now greater in absolutevalue than the supply pressure, evacuates the lens cavity and collapsesthe cornea. In practice the supply water includes entrained air andthere may be bubbles in the supply. The mass of the water and thecompliance of the air line, coupled with the reduced flow resistance asthe occlusion is overcome, act as an under damped oscillatory system inwhich the cornea may be vibrated violently as a function of the size ofthe cavity.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved method and animproved means by which to conduct eye surgery with greater safety;

Still another object is to provide such improvements in a way that isinexpensive, is applicable to all current brands ofirrigation-aspiration systems apparatus and requires less, not more,skill in performing such procedures as cataract removal.

These and other objects and advantages of the invention are realized inpart by the provision of a method and apparatus which convert what is anormally underdamped irrigation-aspiration system to an overdampedsystem when an occlusion is cleared. It can be described as the methodof minimizing corneal collapse during material removal from an eye bythe irrigation-aspiration procedure using irrigation-aspirationapparatus which method comprises the steps of:

sensing an increase in negative pressure in the aspiration line of saidapparatus; and

increasing flow resistance in said aspiration line as an incident tohaving sensed said increase.

Stated another way, the objects and advantages of the invention stem, inpart, from the provision of an article of manufacture for inclusion inthe aspiration line of an irrigation-aspiration apparatus for conductingeye surgery by the irrigation-aspiration method the article comprising aflow tube the walls of which have sufficient renitence to maintain theflow path open at negative internal pressure values corresponding to thepositive internal pressure values of the irrigation line of saidapparatus, said walls being responsive to more negative internalpressures to deform to reduce the cross-sectional area of said flowpath.

A tool is provided in the preferred form which comprises dividing meansfor dividing material found in the interior of an eye and two flow pathseach having openings proximate to one another and to said dividingmeans. The tool further comprises a variable flow resisting elementresponsive to increasing negative pressure to increase it's resistanceto flow connected in series with one of said flow paths.

THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram of a presently preferredirrigation-aspiration apparatus for use in conducting cataract removalby the irrigation-aspiration method;

FIG. 2 is a schematic diagram of a fragment of the tool of the apparatusof FIG. 1 shown as it might appear while in use during cataract removal;

FIG. 3 is a simplified, cross-sectional view of the preferred vacuumcontrolled flow resistor.

FIG. 4 is a graph depicting pressure variations in the eye and in thesupply and aspirating flow paths in a representative situation in theabsence of the invention; and

FIG. 5 is a graph depicting pressure variations in the eye and tool in arepresentative situation during use of the apparatus of FIG. 1 with theimprovement of the invention in place;

FIG. 6 is a perspective view of a preferred form of double flowpathvacuum controlled flow resistor;

FIG. 7 is a cross-sectional view of another form of vacuum controlledresistor; and

FIG. 8 is a cross-sectional view taken on line 8--8 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus of FIG. 1 includes a source of irrigation fluid, abalanced salt in water solution, referred to here as water. It includesa tool for removing material from the inner eye, a supply line arrangedto conduct water under positive pressure from the source to the tool, ameans for developing a negative pressure and an aspiration line arrangedto utilize the negative pressure to conduct water and removed materialfrom the tool. The elements thus far described form anirrigation-aspiration system of conventional form. In this embodimentthe source is the water container 10. The supply line is numbered 12.The tool is numbered 14 and the aspiration line is numbered 16. Thesupply line ends at 18 and the aspiration line begins at 20 in the tool14.

The tool is mounted on a handle which is not shown here both because thehandle per se does not form part of the invention and for the sake ofclarity. The tool performs several functions. It must divide and severthe lens or other material to be removed. To that end it is formed withcutting edges at it's forward extremity at 22. Some tools incorporateultrasonically activated elements to facilitate material division. Thelens or other material is to be removed by aspiration into and along theaspiration line. The vehicle for aspiration is water which is suppliedfrom the source 10 by line 12. The water is supplied at positivepressure by elevating the water container 10. In this case it is mountedon a pole 30. In a typical case it is mounted about 30 inches above theworking level of the tool. The operating surgeon may control the supplypressure by having the bottle elevated or lowered in some degree.Negative pressure for aspiration is developed by any convenient meansand in the case of most systems is developed by connecting theaspiration line to a pump. Peristaltic pumps are commonly used and it issuch a pump that is depicted at 32 in FIG. 1.

The cross-sectional area of each of the supply and aspiration line endsis necessarily small because those ends must be placed in closeproximity to the cutting edges and they and the cutting edges areinserted through an edge opening in the cornea into the lens to beviewed with a microscope during the removal phase of the procedure. Thatis depicted schematically in FIG. 2 wherein the end of the tool isnumbered 24. It extends under the cornea 26 into the lens 28.

The conventional irrigation-aspiration apparatus includes a normallyopen, solenoid actuated clamp valve which may be mounted on the pumpunit. The supply line is placed in the clamp structure such that it ispinched closed when the solenoid is closed by operation of a footswitch. Another valve, responsive to operation of the foot switch andlocated in an air inlet line, opens the aspiration line to atmosphere.They are also the only protection in the conventional system against theconsequences of a step function increase in negative pressure in thecavity from which material is being removed. The primary occasion forsuch an increase is the sudden relief of a blockage of the aspirationtube inlet. If a corneal collapse is observed, the surgeon can pinch offthe water supply, open the aspiration line to relieve the negativepressure, and stop the pump. However, those expedients do not solve theproblem for at least two reasons. In practice the only means availableto the surgeon for clearing a blockage are mechanical manipulation ofthe tool and the suction force at the aspiration line end. To use thesuction force requires closing the air inlet line and operating thepump. Pressure in the eye cavity will drop immediately when theocclusion is cleared and pinching off the line and stopping the pumpwill ordinarily not prevent either the drop or the resulting collapse ofthe cornea. The other reason is that the only means the surgeon has toknown that negative pressure must be reduced is to observe a cornealcollapse which is akin to closing the barn door after the horse is gone.The supply line valve is numbered 70. The air inlet passage to theaspiration line is numbered 72 and it's air inlet valve is numbered 74.

The pressure variations attending an occlusion in the conventionalsystem and the occlusion's relief are described graphically in FIG. 4where y distance represents the magnitude of pressure and the x distancerepresents, in turn, a period prior to an occlusion of the aspirationline input, the time of an occlusion, a period over which it persistsfollowed by the time of sudden relief of the occlusion and, finally, aperiod following relief. The supply and aspiration line ends have smallflow path areas which present relative high resistance to flow. Thatfactor serves to limit the rate of pressure change in the cavity formedby removal of lens material. Typical supply and aspiration lines have aninside diameter up to about 2 millimeters. However, the usual pump iscapable of developing negative pressure up to about 500 millimeters ofmercury to insure that the removed material will be aspirated throughthe small aspiration inlet. The upper dashed line in FIGS. 4 and 5represents the pressure at which water arrives for discharge into thecavity. It's magnitude is determined by the height of the supplycontainer. Accordingly it's magnitude is substantially constant. Thelower dashed line of the graphs represents the negative pressure at theaspirating line inlet downstream from the occlusion and, in the case ofFIG. 5, upstream from the vacuum controlled resistor. The dotted line inFIG. 5 represents pressure in the aspiration line downstream from thevacuum controlled resistor. If adjusted properly, the negative pressureat the aspiration line inlet balances the positive supply pressure sothat cavity pressure, the solid line, corresponds to ambient atmosphericpressure in the absence of an occlusion. When occlusion occurs, flowresistance at the aspiration inlet approaches infinity. The pump pullsdown the pressure in the aspiration line to a value substantially belownormal operating value. The cavity pressure increases somewhat but theincrease is limited because water leaks from the cavity via theincision.

When the occlusion is cleared, the sum of the positive supply pressureand the aspirating line pressure swings sharply negative and water issucked from the cavity. The net negative pressure is overcome by initialinrush of water into the cavity line input so the cavity pressure risesrapidly to become less negative. The effect is to produce a decrementaloscillation of cavity pressure. Oscillation magnitude, frequency anddecay rate depend upon the mass and compliance of the water and thesystem elements and thus, in part, on cavity volume. The pressureexcursions in the cavity effect collapse and flutter of the cornea.

That collapse and oscillation is largely overcome in the invention bythe inclusion of a vacuum responsive, variable flow resistance elementin the aspiration line. Such an element, a vacuum controlled resistor,is included in FIG. 1 where it is identified by the numeral 40. It formspart of the tool 14 in the preferred form. The effectiveness of theelement is greatest when it is close to the aspiration line inlet.

Element 40 may have a variety of forms. Flow resistance in a flow linevaries with cross-sectional area, flow path shape, obstructionconfiguration, length of restrictions and some other factors. It iscurrently preferred to employ a flow section in the aspiration linewhich will deform in response to increase in negative pressure to reducethe cross-sectional area of the flow path and which tends to reform toincrease flow area when aspirating line pressure becomes less negative.The currently preferred element having that quality is depicted in FIG.3 and is the element 40 of FIG. 1. It is shown in cross-section taken ona plane which contains its longitudinal axis. It is an elongatedcylindrical tube having reduced outer wall diameter and it has reducedwall thickness, except at its ends. In relaxed condition the renitenceof the tube forces it to substantially cylindrical shape. Thecross-sectional area is approximately equal to the cross-sectional flowarea, of the remainder of the aspiration line other than at the tool. Inpreferred form the section of reduced wall thickness is at least onequarter of an inch long and more preferably is about one inch long. Thewall thickness in the reduced area is from 0.020 inches to 0.001 inches.It has a cross sectional flow area in relaxed condition between 0.0005to 0.05 square inches and is formed of an inert elastomeric material,preferably a silicon rubber. It should collapse nearly completely atsome negative pressure such that the pressure upstream from the vacuumcontrolled resistor does not exceed a value during an occlusion thatwill draw down the pressure in the eye cavity to about 50 millimeters ofmercury below atmospheric pressure after the occlusion is cleared. Thecornea may collapse if the negative pressure exceeds the value.Selection of the response characteristics is based on balancing the needfor sufficient negative pressure at the cavity to overcome occlusionsagainst the advantage of limiting the negative pressure surge onclearance by increasing aspiration line flow resistance. It is possibleto find a workable design by conventional computation methods but only aminimum of experimentation is needed to find an entirely suitable wallthickness and reduced section length for any commercially availableelastomeric tubing. If the vacuum controlled resistor is allowed toclose the aspiration line completely, suction at the aspiration lineinlet will not increase further. If the occlusion has not been cleared,suction is no longer available to clear it. The line must remain open insome degree to achieve clearance. As long as the line remains open insome degree, suction force is available by Pascal's Law to help clearthe occlusion. The ideal element is one that collapses readily undernegative pressure greater in absolute value than the positive supplypressure but which does not close the flow path completely unless theblockage is so severe that unduly high negative pressure would bedeveloped upstream from the element. That complete closure feature neednot be incorporated in the design of the element but it is included inthe preferred embodiment as a gross safety measure. The pump controllercan be arranged to cease pumping if negative pressure becomes excessive.Complete closure of the resistance element is then useful only in theevent of failure of the pump controller to limit suction.

FIG. 5 illustrates that effect of the negative pressure responsive,variable resistance of the invention is to convert the system that isnormally underdamped at the eye cavity to an overdamped systemimmediately on release of an occlusion. Again the pressure of theirrigation fluid, the supply water, is substantially constant before,during and after overcoming an occlusion with aspiration pressure.Following an occlusion, pressure in the cavity increases but theincrease is slight because the water escapes at the incision from thespace under the cornea. When an occlusion occurs, the pump "draws down"the aspiration line. The degree in which the pressure downstream fromthe cavity is permitted to go negative is determined by the negativepressure at which the variable resistance device, element 40, shuts offcommunication in the line. Comparison of FIG. 5 with FIG. 4 shows thatthe variable resistance device 40 has limited the negative pressure ofthe aspiration line upstream from the device to a lesser value in FIG.5. When the occlusion is cleared in FIG. 5, the pressure of water in theeye cavity and the renitence of the variable resistance element tend toforce the flow path of the variable resistance device to increased areaand lower resistance. However, negative pressure downstream toward thepump tends to draw the device's walls to smaller area. Consequently, thedevice presents a considerably higher flow resistance at the beginningof the transition period which slowly decreases to a smaller value asthe fluid flows through it. The net result is reduced rate of pressurechange in the eye cavity and a much smaller and non-oscillatory negativepressure excursion and that translates into minimal corneal collapse.

The scale lines and the dashed cavity pressure lines in FIGS. 4 and 5correspond substantially to oscilloscope scale lines and traces recordedduring actual test of a system with the vacuum controlled resistor inthe case of FIG. 5 and without it in the case of FIG. 4. The bottle wasmounted 70 centimeters above the tool. The supply and aspiration lineshad an inside diameter of 2 millimeters. The vacuum controlled resistorhad a relaxed inside diameter of 0.06 inches. It had a wall thicknessreduced to 0.005 inches over a length of one inch and was made of mediumhardness, silicone rubber flow tube. It was located in series in theaspiration line eight inches from to aspiration line inlet. In FIGS. 4and 5 the interval between adjacent vertical scale lines represents onevolt or 40 millimeters of mercury. The interval between horizontal scalelines is 0.2 seconds. In both Figures the occlusion was cleared at 0.4seconds. In FIG. 4 the eye cavity pressure dropped to about negative 90millimeters of mercury. In FIG. 5, inclusion of the vacuum controlledresistor limited the drop in the eye cavity to less than 20 millimetersof Mercury. In FIG. 4, the undulations in the curve after 1 secondresult from pressure variation produced in the aspiration line by theperistaltic pump. Inclusion of the vacuum controlled resistor smoothesout those variations as shown by the solid line in FIG. 5.

In the experiment depicted in FIG. 5, the negative pressure in theaspiration line upstream from the vacuum controlled resistor reachedabout 250 millimeters of mercury. Increasing the wall thickness of theresistor to 0.006 inches resulted in an reduction of the peak pressureto negative 300 millimeters and an increase to a wall thickness to 0.007inches resulted in a peak pressure reduction to about 350 millimeters ofmercury. The ratio of change in peak negative pressure to wall thicknesschange can be reduced substantially, and the wall thickness dimensionmade less critical, by connecting two or more vacuum control resistorsin parallel as shown in FIG. 6. In some cases the parallel arrangementmay be preferred.

Another variation is shown in FIGS. 7 and 8. Here the exterior of thevacuum controlled resistor 300 is subjected to the pressure in a chamber302 which is included in series in the water supply line 304. By thisarrangement, the effect of the vacuum controlled resistor is made moreuniform despite variation in the height of the supply water container.The degree in which flow resistance is increased is a joint function ofthe negative pressure in the aspiration line and the positive pressurein the supply line.

The particular embodiments described herein and which are shown in thedrawings represent what are considered to be the best embodiments andmodes of practicing the invention. However, it is to be understood thatother embodiments and modes of practicing the invention are possible andthe scope of the invention is not to be considered to be limited to whatis shown and specifically described is to be limited instead by thescope of the appended claims. In this connection the term "vacuumcontrolled resistor" is intended to be a generic description rather thana designator of only the particular form shown and described.

We claim:
 1. A system for performing surgery on an eye, wherein thesurgery produces material, comprising:a supply line for providing afluid to the eye; an aspiration line for removing said fluid and thematerial from the eye, said aspiration line having a first end near theeye and a second end opposite from said first end; and a flow tubehaving a fluid resistance and being connected to said aspiration linebetween said first and second ends of said aspiration line, said flowtube having a wall constructed to contract when the material from thehuman body causes the fluid pressure within said aspiration line todecrease to a predetermined value, wherein said flow tube fluidresistance increases such that said first end of said aspiration linewill have a greater fluid pressure than said second end of saidaspiration line.
 2. The system as recited in claim 1, wherein said flowtube collapses when the fluid pressure within the flow line decreases toa predetermined value.
 3. The system as recited in claim 1, wherein saidflow tube has a cross-sectional area between 0.0005 and 0.05 inches anda wall thickness over a length greater than 0.15 inches which is between0.02 and 0.001 inches.
 4. The system as recited in claim 1, wherein saidflow tube is surrounded by a chamber in fluid communication with saidsupply line, such that said wall movement is dependent upon saidaspiration line and said supply line fluid pressures.
 5. An apparatuswithin a flow line having a first point and a second point, the flowline having a fluid that flows from the first point to the second point,comprising:a flow tube having a fluid resistance and being connected tothe flow line between the first and second points of the flow line, saidflow tube having a wall constructed to collapse when the fluid pressurewithin the flow line decreases to a predetermined value, such that saidflow tube resistance increases, said flow tube further having across-sectional area between 0.0005 and 0.05 inches and a wall thicknessover a length greater than 0.15 inches which is between 0.02 and 0.001inches.
 6. An apparatus within an aspiration line of a system used toremove material from a human body, the aspiration line having a fluidthat flows from a first point in the aspiration line to a second pointin the aspiration line, comprising:a flow tube having a fluid resistanceand being connected to the aspiration line between the first and secondpoints of the aspiration line, said flow tube having a wall constructedto contract when the fluid pressure within the aspiration linedecreases, such that said flow tube fluid resistance increases, saidflow tube further having a cross-sectional area between 0.0005 and 0.05inches and a wall thickness over a length greater than 0.15 inches whichis between 0.02 and 0.001 inches.